In his book titled "Manufacturing Techniques for Surface Mount Assemblies" author R.J. Klein Wassink describes the relationship between solder land dimensions, component lead coplanarity, board warpage, solder volumes, and solder paste flux wetting behaviors. Mr. Wassink provides the following formula for calculating the necessary solder volumes for a given lead coplanarity: (Wp/W) x d x Hs x f(sh) = finished solder height

Wp = Solder paste stencil width W = PCB solder pad width d = Stencil thickness Hs = Volume fraction of solder in paste ( Hs is about 55% with 90% solder metal) f(sh) = Shape factor = the height h of the circle segment with cord of W divided by the thickness d1 of the rectangle with the same area and same length W. The factor is about 1.4. This formula is used to calculate finished solder height which when used with Wassink's rule, "that a maximum non-coplanarity of less than 75% of the finished solder height, on the copper land provides a condition for stable and robust processing" will show how much non-coplanarity your process can handle. For a .020" pitch QFP, which uses a PCB solder pad width of .012", stencil width of .011", and stencil thickness of .006", the calculations show the following:

.011/.012 x .006 x .55 x 1.4 = .0042" finished solder height. .0042" x .75 = .0032" max. lead non-coplanarity for robust processing.

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